Granulating processes, which coagulate polymer-containing latices and then recover the resultant coagulum, are usually required in order to recover desired graft copolymers from polymer-containing latices prepared by emulsion polymerization. In the granulating processes, it is required that aggregates having the powder properties such as particle size distribution, fine particle content, and coarse particle content are stably produced in a large amount. If the aggregates having excellent powder properties can be obtained, troubles such as clogging in line resulting from the aggregates can be prevented.
Here, problems in the conventional methods for manufacturing coagulated particles from a latex prepared by emulsion polymerization are explained. In conventional methods, a graft copolymer is recovered in the form of powder particles from a polymer latex prepared by the emulsion polymerization by the following procedure: Water is added to the latex to adjust a polymer solid content to 10% by weight or less, and then a coagulant is added to the resulting mixture at a temperature sufficiently lower than the softening temperature of the polymer to form coagulated polymer particles. Next, the resulting mixture is heated to at least the softening temperature of the polymer to produce a slurry, followed by dehydrating and drying. However, this process has the following problems: Since the powdered polymer particles have no regular shape and contains a large amount of fine powder, troubles frequently occur during the process or a working environment becomes worse because of dust generation. Further, there is a problem that a bulk specific gravity of coagulated particles is decreased.
Therefore, various improved granulating processes such as a method for coagulating the latex containing the polymer in gas-phase, a method for coagulating the latex containing the polymer in liquid-phase using an organic solvent (for example, see Patent Document 1), a method for softly coagulating the latex containing the polymer in liquid-phase (for example, see Patent Document 2), and a method for crushing coagulated particles in liquid-phase have been developed.
For example, in the method for coagulating the latex containing the polymer in gas-phase, there is a problem that the particle size of the resultant coagulated particles is uneven, and the fine particle content increases, since the latex prepared by emulsion polymerization is dropped from the gas-phase, and a solution containing a coagulating agent is sprayed to the latex, to coagulate the latex.
Patent Document 1 discloses a technique that polymerized particles like spherical shape are prepared by adding a latex prepared by emulsion polymerization to a medium containing an organic solvent under the stirring. However, in the technique of Patent Document 1, there is a problem that powder properties of coagulated particles are unsatisfactory and effects are provided on environment in some cases due to the remaining of the organic solvent in the polymerized particles (for example, core elastic part).
Patent Document 2 discloses a technique for manufacturing coagulated particles like spherical shape containing steps of contacting a latex prepared by emulsion polymerization with a solution containing a coagulating agent under stirring, to subject the latex to soft coagulation, and of further coagulating the softly coagulated latex with a solution containing a coagulating agent. However, in the technique of Patent Document 2, there is a problem that it is necessary to find an appropriate combination of two kinds of solutions containing a coagulating agent such that the differences of the physical properties of coagulated particles can be produced, that is, the coarse particle content increases in the case of decreasing the fine particle content even if the particle diameter is controlled by adjusting pH, the concentration of the solution containing a coagulating agent at the time of coagulation. In addition, there is a problem that water content is high and coagulated particles having a desirable properties can be hardly obtained even if the coagulated particles are subjected to dehydration step.
Patent Document 3 discloses a technique for manufacturing graft polymer particles containing step of discharging a polymerized latex from a given nozzle to contact the latex with a solution containing a coagulating agent. However, in the technique of Patent Document 3, there is a problem of the increase of the fine particle content, and the decrease of a bulk specific gravity, since the viscosity of the polymerization latex becomes lower, and softly coagulated particles having high void are formed due to the incorporation of water into the latex.
In spite of such intensive efforts, such processes are unsatisfactory even now with respect to productivity, powder properties, and facility costs; hence, the development of new granulating processes has been desired.
Further, in order to improve these problems, a method for using the coagulating agent of the polymer is proposed as a new method for granulating the latex containing the polymer (for example, Patent Document 4). The method granulates the latex prepared by emulsion polymerization by adding an anionic polyacrylamide as the coagulating agent of the polymer and the inorganic salt to the latex prepared by emulsion polymerization.
However, in the technique of Patent Document 4, when the solid content of the latex prepared by emulsion polymerization is more than 10% by weight, the latex is not stirred with the coagulating agent, and the appropriate coagulated particles cannot be obtained. Therefore, in order to obtain the appropriate coagulated particles, it is necessary that the solid content is adjusted to 10% by weight or less by diluting the latex prepared by emulsion polymerization with a large amount of water, so that the burden for draining waste water becomes much. In addition, the granulating operation cannot be carried out at only higher temperature of 80° C. regardless of the nature of the latex prepared by emulsion polymerization, so that the technique is not satisfactory in the view of the consumption of large energy.
Further, in the case of using the coagulating agent of the polymer in Patent Document 4, there is a problem of the decrease of the fine particle content having 60 mesh or less (sieve opening of 250 μm or less), and the increase of the coarse particle content having 4 to 20 mesh (sieve opening of 4.75 mm to 0.85 mm).